The present invention relates to a hermetically sealed injection semiconductor laser device, and more particularly to such a device in which a crystal layer pellet and a lens element are disposed within a sealed hollow cylinder.
The incorporation of a double heterostructure into an injection semiconductor laser has made possible continuous oscillation at room temperature. The life expectancy of this type of laser device has been much improved by recent technical developments, and semiconductor lasers capable of continuous oscillation at room temperature for more than 10,000 hours, with a light output on the order of 10 mw may now be achievable. Contemporaneously with the development of various stripe structures, it has become feasible to efficiently control the mode of laser light. Under the circumstances, the advent of practicable and easily operable injection semiconductor laser devices has been awaited. However, a problem that remains to be solved, is the construction of the casing for the laser elements.
The semiconductor laser permits high-speed direct-modulation at frequencies on the order of gigahertz, and the output laser beam is emitted in two opposite directions unless special provisions are made to prevent this. One known, commercially available package such as the RCAOP-3 made by RCA Corp. has a structure similar to a well-known transistor cap. (The RCAOP-3 has a glass top, instead of metal.) This structure, however, has very significant practical disadvantages. For example, it has an elongated terminal through which current is supplied, resulting in increased inductance and capacitance which degrades its high frequency characteristics. Furthermore, in this structure, the output laser beam is derived from only one side. On the other side, the laser beam is scattered as waste or totally reflected back by a fully reflective film coating on the end of the laser crystal. It is more desirable that the output laser beam be used in both directions. For example, one laser beam can be utilized for monitoring purposes. When the light output is suitably fed back to the drive current circuit, one laser beam can be used to suppress output variations attributable to changes in ambient temperature, characteristic deterioration or other factors. The two output laser beams can be used in combination, or a laser amplifier can be formed when one laser beam is used as an input, and the other as an output in an amplifying stage maintained by suitably coating the laser crystal with antireflective film.
Another known casing structure is arranged such that the laser crystal is directly epoxy-molded as in a light-emitting diode. This structure, however, has not as yet achieved sufficient reliability and heat-dissipation to be of practical utility.
The injection semiconductor laser emits its output laser beam at a relatively large angle of divergence. For practical applications, this divergent laser beam must often be converted into a parallel beam by an optical system, as in the ordinary gas laser and solid-state laser, or into a convergent beam to be adapted to glass fiber optical transmission cables. In the prior art, no substantial improvements have been made on this point.